The research proposed here investigates the physiologic mechanisms which are important in maintaining oxygen transport at high altitudes. The project is designed to identify both the genetically based adaptive mechanisms which may be present in species native to high altitude and the phenotypic acclimatization responses which may be present in lowland species, but only expressed following exposure to conditions of ambient hypoxia. The specific objective of the proposed research is to determine the relationship between ambient hypoxia and tissue capillarity. The hypothesis to be tested is that there are changes at the tissue level to potentiate oxygen diffusion from the blood to the sites of oxidative metabolism; namely, a reduction in the diffusion distance via an increased tissue capillarity. We further propose that the adaptive response is primarily, or entirely, seen in the embryo. The research proposed here will provide baseline information on tissue capillarity and the importance of increases in tissue capillarity in maintaining tissue oxygenation during exposure to ambient hypoxia.